1. Field of the Invention
This invention generally relates to extractive gas sampling systems such as are used for analysis of process gases or fossil fuel combustive gases being vented through a stack.
2. Description of the Prior Art
An important category of extractive gas sampling relates to the compliance monitoring requirements enforced by the United States Environmental Protection Agency (EPA). Many sources of air pollution, such as fossil-fueled power plants, incinerators, metal smelters, and cement kilns, are required to monitor levels of certain gaseous species that are released into the atmosphere. These species include sulfur dioxide, nitrogen oxides, carbon monoxide, carbon dioxide and oxygen. The EPA standards for compliance monitoring systems are delineated in Volume 40 of the Code of Federal Regulations.
The gas streams to be monitored typically have certain intrinsic characteristics which complicate testing. For example, they generally contain 6% to 20% by volume of evaporated moisture, which results in a sample dew point well above that of normal ambient temperatures. Also, the gas streams often contain significant amounts of condensed moisture in the form of entrained water droplets and fog. Acid gases, such as sulfur dioxide are also generally present. Additionally, the gas streams invariably contain large quantities of particulate debris such as soot, fly-ash from fossil fuels and process material.
In order to analyze a sample for its gaseous constituents, it is necessary to remove the particulates and transport the sample to a remote location suitable for the operation of gas analysis instrumentation. For accurate measurements and for reliability of the test equipment, it is necessary to ensure that moisture and gases will not condense either in the sample probe, the sample lines, or the analyzers. However, the methods used to accomplish these goals must not themselves alter the samples in a way that negatively impacts testing accuracy.
In the past, two basic types of sampling systems have been developed for analysis of gaseous mixtures. The first type, the traditional extractive system, is shown in FIG. 1. Many vendors have supplied similar systems over the years. This system, however, has proved to have many undesirable drawbacks as described below. The second type, illustrated in FIG. 2, is a venturi dilution probe system. This type of system was developed in the 1980's primarily in response to perceived inadequacies with the traditional system. As discussed more fully below, however, the venturi probe system is also not without disadvantages.